Roy;
I presume this is probably a lot of the same mat'l as found in Burrard's The Modern Shotgun. He does in this work show a barrel which received three bulges from one shot. The obstruction was not quite severe enough to cause a burst, but as it moved down the barrel the Wave pressure bounced back to the breech, forward again until it caugth back up, creating the second bulge & eventually the third. Each successive bulge was smaller than the previous one.
One often hears how "OutDated" Burrard's work is, but after having studied it for many years I am of the conclusion that except for the substitution of Plastic for paper & felt, not much has truly changed.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra